1. Field of the Invention
The invention relates to a method for measuring a characteristic impedance and a propagation constant of a two-port network, in which method at least two input impedances are measured by means of a reflection measuring instrument at an input port of the two-port network with at least two different predetermined terminations of an output port of the two-port network and from the at least two input impedances a characteristic impedance and a propagation constant are determined.
2. Discussion of Background
In the transmission of radio-frequency signals, it is absolutely necessary that the characteristics of the transmission link be accurately known in order to be able to match terminal devices (for example transmitters and receivers) and transmission link to one another. This is the only way in which reliable signal transmission can be ensured.
It is known that the characterisics of a linear two-port network are essentially given by the two quantities of characteristic impedance and propagation constant. These quantities can be determined, for example with the aid of a reflection measurement. A corresponding method is disclosed, for example, in the printed document "Durchgangsdampfung--ermittelt uber Reflexionsmessung" (transmission loss--determined by reflection measurement), Neues von Rohde+Schwarz 118, summer 1987. In this method a signal wave is coupled via an input port into the two-port network to be measured, passes through the two-port network, is totally reflected at an output port and returns to the input port of the two-port network where it is then measured. In this manner, two input impedances are measured, the total reflection at the output port being determined, on the one hand, by a short-circuit and, on the other hand, by an open circuit.
The advantage of the reflection measurement lies in the fact that the signal wave is measured at the same location where it is generated. Since, in addition, only an input port of the two-port network needs to be connected to the measuring instrument, for example installed cables can also be measured without difficulties.
However, problems occur in the known measuring method if a frequency range of 100 MHz and more is to be used as operating frequency. This is because it has been found to be difficult to implement good short circuits or open circuits, respectively, at such frequencies. As a consequence, the total reflection is disturbed and the measurement results become very inaccurate.